Next hop chaining for forwarding data in a network switching device

ABSTRACT

A route for a data unit through a network may be defined based on a number of next hops. Exemplary embodiments described herein may implement a router forwarding table as a chained list of references to next hops. In one implementation, a device includes a forwarding table that includes: a first table configured to store, for each of a plurality of routes for data units in a network, a chain of links to next hops for the routes; and a second table configured to store the next hops. The device also includes a forwarding engine configured to assemble the next hops for the data units based on using the chain of links in the first table to retrieve the next hops in the second table and to forward the data units in the network based on the assembled next hops.

RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.12/195,686, filed Aug. 21, 2008, now U.S. Pat. No. 8,014,317, the entirecontents of which is incorporated herein by reference.

BACKGROUND

Current networks typically include devices, such as routers, switches orgateways, which transfer or switch data, such as packets, from one ormore sources to one or more destinations. A packet is one format of datain which encapsulated data can be transmitted through a network based oncontrol information that is stored in a header portion of the packet. Arouter is a switching device that can receive a packet and, based on thepacket header, may forward the packet towards its final destination.

Existing routers include forwarding engines for receiving and forwardingincoming packets to their intended destinations. To forward incomingpackets from an input port to an appropriate output port, routers mayperform complex data manipulation actions on the packet header. Suchdata manipulation actions frequently result in the router rewriting aportion of the packet header before transmitting the packet to anappropriate output port of the router.

SUMMARY

In accordance with one implementation, a method may include receiving adata unit and retrieving a plurality of next hops associated with thedata unit based on a chained representation of the next hops in aforwarding table. The method may further include rewriting a header ofthe data unit based on the retrieved plurality of next hops andforwarding the data unit in the network based on the retrieved pluralityof next hops.

Another aspect is directed to a network device that includes a routingengine configured to receive network topology information from a networkand to, based on the network topology information, generate next hopsthat are relevant to routes for data units through the network. Thedevice further includes a forwarding table stored in a memory andincluding a first portion of the forwarding table that stores, for eachof a plurality of routes in the network, links to next hops for theroute, and a second portion of the forwarding table that stores the nexthops. The device further includes a forwarding engine configured toassemble the next hops for a data unit based on a lookup of the links inthe first portion of the forwarding table in the second portion of theforwarding table and to forward the data unit in the network based onthe assembled next hops.

Yet another aspect is directed to a device comprising a forwarding tableand a forwarding engine. The forwarding table includes a first tableconfigured to store, for each of a plurality of routes for data units ina network, a chain of links to next hops for the routes, and a secondtable configured to store the next hops. The device also includes aforwarding engine configured to assemble the next hops for the dataunits based on using the chain of links in the first table to retrievethe next hops in the second table and to forward the data units in thenetwork based on the assembled next hops.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate one or more embodiments describedherein and, together with the description, explain the invention. In thedrawings,

FIG. 1 is a diagram of an exemplary network;

FIG. 2A is a diagram of a network node according to an exemplaryimplementation;

FIGS. 2B, 2C and 2D are diagrams of exemplary paths data units may takewhen traversing the exemplary network node of FIG. 2A;

FIG. 3 is a diagram of an exemplary configuration of an interface of thenetwork node of FIG. 2A;

FIG. 4 is an exemplary diagram of a data unit that may traverse thenetwork of FIG. 1 from a source to a destination;

FIG. 5 is a diagram of a forwarding table according to an exemplaryimplementation;

FIG. 6 is a diagram illustrating an exemplary implementation of aforwarding table using next hop chaining;

FIG. 7 is a flowchart of an exemplary process for forwarding data unitsbased on a chained representation of composite next hops;

FIG. 8A is a diagram illustrating exemplary routes associated withtwelve exemplary packets;

FIG. 8B is a diagram illustrating an exemplary next hop table;

FIG. 8C is a diagram illustrating an exemplary next hop logical chaintable;

FIG. 9A is a diagram illustrating three exemplary routes associated withEthernet over ATM transmission.

FIG. 9B is a diagram illustrating an exemplary next hop tableconstructed based on next hop rewrites;

FIG. 9C is a diagram illustrating an exemplary next hop logical chaintable;

FIG. 10A is a diagram illustrating exemplary routes;

FIG. 10B is a diagram illustrating an exemplary next hop tableconstructed based on next hop rewrites;

FIG. 10C is a diagram illustrating an exemplary next hop logical chaintable.

FIG. 11A is another diagram illustrating exemplary routes;

FIG. 11B is a diagram illustrating a series of routes represented usinga flat, non-chained representation for next hops;

FIG. 11C is a diagram illustrating an exemplary next hop table based onthe routes shown in FIG. 11A; and

FIG. 11D is a diagram illustrating an exemplary next hop logical chaintable for the routes shown in FIG. 11A.

DETAILED DESCRIPTION

The following detailed description refers to the accompanying drawings.The same reference numbers may be used in different drawings to identifythe same or similar elements. Also, the following detailed descriptiondoes not limit the invention. Instead, the scope of the invention isdefined by the appended claims and equivalents.

A route for a data unit through a network may be defined based on anumber of next hops. Exemplary embodiments described herein mayimplement a router forwarding table as a chained list of references tonext hops. In contrast to a flat forwarding table, where there is asingle next hop associated with each route, and that single next hop'srewrite contains the entire re-write transformation performed on headersof data units corresponding to the route, using a chained list ofreferences to next hops can reduce the memory requirements to store theforwarding table and can lead to a reduced time to update the state ofthe forwarding table in response to a network disturbance.

A “next hop,” as this term is used herein, may include an elementaryaction performed for a packet as it transits a router. Examples ofelementary actions include replacing a layer 2 (L2) header of a dataunit, performing a layer 3 (L3) action (e.g., network addresstranslation (NAT)) on the data unit, or making a copy of the data unit(replication for sampling)). Some packets may also be associated withmultiple network hops, referred to as a “composite next hop,” where thisterm refers to a commonly referenced collection of other next hops(e.g., a collection of next hops) and a function to be performed onthose next hops (e.g., C=F(N1, N2, . . . Nn), where C is the compositenext hop, F is the function and {N1, N2, . . . , Nn} is the collectionof next hops that the function is acting upon). Examples of the function(F) may include (but is not limited to): 1) perform any one of theactions of the next hops in the composite next hop; 2) perform all ofthe actions sequentially of the next hops in the composite next hop; 3)perform the actions of the next hops until some condition CON evaluatesto true (alternatively to false) and then transit the data unit, or takea different set of actions; or 4) make copies of the data unit andperform the action on all of the copies.

Exemplary System

FIG. 1 illustrates a system 100 according to an exemplary embodiment fortransmitting data units between a source node 110 and a destination node120. A “data unit,” as this term is used herein, may represent a packetor another fixed or variable length unit of data (e.g., a frame).

System 100 may include a source node 110 and a destination node 120connected via a network 130. Source node 110 and destination node 120may each include, for example, a server or a client computing device.Source node 110 and destination node 120 may connect with network 130via wired, wireless or optical connection links. Network 130 may includeone or more networks of any type, including a local area network (LAN),metropolitan area network (MAN), wide area network (WAN), Internet, orIntranet. Network 130 may include any number of network nodes forrouting data units through network 130, with multiple nodes 140-1through 140-Q (generically and individually referred to herein as a“node 140-x”) shown in FIG. 1 by way of example. Each network node 140-xmay include a network device, such as, for example, a router, a switch,or a gateway, that routes or switches incoming data units towards one ormore destinations. Data units that originate at source node 110 may berouted to destination node 120 via multiple ones of nodes 140-1 through140-Q.

The number and configuration of nodes depicted in FIG. 1 is forillustrative purposes only. A network may include different, more orfewer nodes than are illustrated in FIG. 1. Additionally, only a singlesource node 110 and destination node 120 have been shown in FIG. 1 forpurposes of simplicity. However, multiple source nodes and destinationnodes may be coupled to network 130.

In general, data units sent by source node 110 and destined fordestination node 120 may traverse network 130 by passing from one nodeto another until the data unit has reached its destination. As the dataunit traverses network 130, each node 140-x that receives the data unitmay make a decision as to which node the data unit should next beforwarded. When routing data through a network such as network 130,nodes in the network may follow a routing protocol that attempts tomaximize the efficiency of the network by choosing the “best” route forthe data unit through network 130.

Exemplary Network Node

FIG. 2A illustrates a network node 140-x according to an exemplaryimplementation in which network node 140-x is a router. Network node140-x may receive one or more data units from one or more incomingphysical links, process the data units to determine destinationinformation, and transmit the data units on one or more outgoing linksin accordance with the destination information or one or more otherproperties of the data units. The data units may include packets, cells,or datagrams; fragments of packets, cells, or datagrams; or other typesof data. The one or more incoming physical links and the one or moreoutgoing links may connect to other nodes or edge devices in network130.

Network node 140-x may include one or more ingress interfaces 200-1through 200-N (generically and individually referred to herein as an“ingress interface 200”), a switch fabric 210, a routing engine 220, andone or more egress interfaces 230-1 through 230-M (generically andindividually referred to herein as an “egress interface 230”). Eachingress interface 200 may receive data units from network 130 via one ormore physical links and may forward the received data units throughswitch fabric 210 to a respective egress interface. Each ingressinterface 200 may forward received data units to a respective egressinterface 230 using forwarding tables received from routing engine 220.Routing engine 220 may communicate with other of nodes 140-1 through140-Q connected to network node 140-x to exchange information regardingnetwork topology. Routing engine 220 may create routing tables based onthe network topology information and forward the routing tables to eachingress interface 200 and each egress interface 230. Routing engine 220may also perform other general control and monitoring functions fornetwork node 140-x.

Switch fabric 210 may include one or more switching planes to facilitatecommunication between ingress interface 200-1 through 200-N and egressinterfaces 230-1 through 230-M. In one exemplary implementation, each ofthe switching planes may include a three-stage switch of crossbarelements. Other types of switching planes may, however, be used inswitch fabric 210. Egress interfaces 230-1 through 230-M may receivedata units from switch fabric 210 and may forward the data units towardsdestinations in the network via one or more outgoing physical links.

Network node 140-x may include fewer, additional and/or differentcomponents than are shown in FIG. 2A. For example, network node 140-xmay include one or more intermediate service cards (not shown) thatconnect to switch fabric 210 and which may include special purpose cardsthat perform various specialized functions. For example, theintermediate service cards may include a firewall service card, anintrusion detection service card, an encryption/decryption service card,or other types of special purpose cards.

The interfaces in FIG. 2A have been arbitrarily designated as “ingressinterfaces” or “egress interfaces” for purposes of illustration. Eachone of interfaces 200-1 through 200-N or 230-1 through 230-M may act asan ingress and/or an egress interface. Thus, each one of interfaces200-1 through 200-N or 230-1 through 230-M may receive data units fromthe external network, or forward data units received from switch fabric210 towards destinations in network 130.

FIGS. 2B, 2C and 2D illustrate examples of interfaces 200 and 230 actingas both ingress and egress interfaces, and examples of paths throughnetwork device 140-x that includes interfaces 200 and/or 230. As shownin the example of FIG. 2B, a data unit 240 may be received at aninterface 200-2, acting as an ingress interface, from external network130 and data unit 240 may be forwarded to an interface 230-1, acting asan egress interface, via switch fabric 210. Interface 230-1 may forwarddata unit 240 towards its destination in the external network 130 via anoutgoing link.

As further shown in the example of FIG. 2C, a data unit 240 may bereceived at an interface 230-M, acting as an ingress interface, fromexternal network 130 and data unit 240 may be forwarded to an interface200-1, acting as an egress interface, via switch fabric 210. Interface200-1 may forward data unit 240 towards its destination in externalnetwork 130 via an outgoing link.

As also shown in the example of FIG. 2D, data unit 240 may be receivedat interface 200-1, acting as an ingress interface, from externalnetwork 130. Interface 200-1 may then forward data unit 240 towards itsdestination in external network 130 via the same link from which thedata unit was received. Interface 200-1, thus, in this example, acts asboth an ingress interface and an egress interface for the same dataunit.

Exemplary Interface

FIG. 3 illustrates exemplary components of an interface 200/230. Asshown in FIG. 3, interface 200/230 may include a forwarding engine 310,a forwarding table 315, and one or more queues 320. FIG. 3 furtherdepicts routing engine 220 being connected to interface 200/230. Routingengine 220 may supply the contents of forwarding table 315 to interface200/230 based on network topology information.

Forwarding engine 310 may receive data units from switch fabric 210, orfrom a physical link connected to network 130 and may inspect one ormore properties (e.g., information contained in a header) of each dataunit, retrieve next hop information from forwarding table 315, rewrite(modify) portions of the header of each data unit based on the next hopinformation, and forward the data unit to the next node in network 130.Queue(s) 320 may be used to buffer data units before they are sentthrough switch fabric 210 or to an external link. Interface 200/230 may,in some implementations, include fewer, additional or differentcomponents than those shown in FIG. 3.

Exemplary Data Unit

FIG. 4 illustrates an exemplary data unit 240 that may originate atsource node 110 and may be sent to destination node 120 via multipleones of nodes 140-1 through 140-Q of network 130. Data unit 240, asshown in FIG. 4, depicts exemplary contents of the data unit at anintermediate node (e.g., one of nodes 140-1 through 140-Q) of network130 as data unit 240 is being forwarded node by node towards destinationnode 120. Data unit 240 may include, among other data, a header 410 anda payload 420. Header 410 may include a source address 430 and adestination address 440. The source address 430, destination address440, or other portions of header 410 may include information relating toa number of different protocols, possibly at a number of differentlayers of a networking model. For example, header 410 may contain layer2 and layer 3 information of the Open Systems Interconnection (OSI)seven layer model.

Header 410 may be rewritten at one or more of nodes 140. Thus, each node140 that receives data unit 240 may examine header 410 and rewrite oneor more portions of header 410 before sending data unit 240 to the nextnode in network 130.

Source and destination addresses, either as received by node 140 and inheader 410, or after processing by node 140, may be represented as oneor more next hops that are relevant to the data unit's route throughnetwork 130. The next hops may, in some situations, be represented as acomposite next hop that includes a function that is to be applied to thenext hop(s).

Exemplary Forwarding Table

FIG. 5 is a diagram conceptually illustrating forwarding table 315according to an exemplary implementation. Forwarding table 315 mayinclude, for each possible destination or route 510 of data unitsreceived by the node, one of more next hops (or a composite next hop)that may be used to determine the egress interface(s) 230 for the dataunit and the information used to rewrite the header of the data unit,called the “next hop rewrite” herein. Destination/route 510 may beobtained based on the header of the incoming data unit and may definethe final destination, as well as possibly the path (route) that thedata unit is to take to get to the final destination. Forwarding engine310 may use the next hops stored in forwarding table 315, and, based onthe composite next hop, may obtain the next hop rewrite for the dataunit.

A number of sets of next hops are shown in FIG. 5. For example, in FIG.5, next hop 502 is shown as a single next hop while next hops 504 and506 include multiple next hops. More specifically, next hop 502 isdefined by the next hop labeled NH₁, next hops 504 are defined as thenext hops labeled NH₂, NH₃, and NH₄, and next hops 506 are defined asthe next hops labeled NH₄ and NH₅.

As previously mentioned, each next hop, such as next hops NH₁, NH₂, NH₃,NH4, and NH₅ may represent an elementary action that is to be performedon the data unit. A composite next hop could also be defined as one ormore next hops and a function that is to be performed on them. The nexthops and/or next hop composition defined in forwarding table 315 may beused by forwarding engine 310 to generate the next hop rewriteinformation for the data unit. The next hop rewrite information maydefine, for example, rewriting of layer 2 and layer 3 information in theheader. For example, when an incoming destination/route 510 correspondsto next hops 504, the header of the data unit corresponding todestination/route 510 may be rewritten to include rewrite informationfor each of next hops NH₂, NH₃, and NH₄.

Existing forwarding tables may be physically constructed as a “flat”forwarding table in which each destination/route 510 corresponds to asingle next hop rewrite action. In such a flat forwarding table, nexthop NH4, for example, may be physically stored in the memory used tostore next hops 504 and the memory used to store next hops 506. Such aflat forwarding table can require a relatively large memory and can takea relatively long time to update when there is a network disturbance,since if NH4 is replaced by a new next hop, e.g., NH10, all of these‘copies’ of NH4 in next hops 504, 506, etc., will be updated to NH10.

Consistent with aspects described herein, forwarding table 315 may beconstructed as a chained data structure in which individual next hops inforwarding table 315 may be stored as a link to the next hop. A fullnext hop rewrite may be formed by the assembled chain of next hops,which may each correspond to a partial rewrite. The next hop chainingmay be implemented as a composition function for a composite next hop.In this manner, next hops for a data unit may be evaluated fromforwarding table 315 as links to a chain of individual next hops toobtain the next hop rewrite for the data unit.

FIG. 6 is a diagram illustrating an exemplary implementation offorwarding table 315 using next hop chaining In this implementation,forwarding table is implemented using a next hop logical chain table 620and a next hop table 630. Next hop logical chain table 620, instead ofstoring the actual values associated with each of the next hops (i.e.,NH₁, NH₂, . . . NH₅) may instead store a link to the next hops. In nexthop logical chain table 620, link A is a link to NH₁, link B is a linkto NH₂, link C is a link to NH₃, link D is a link to NH₄, and link E isa link to NH₅. Each entry (row) in next hop logical chain table 620 maybe indexed by a destination/route 510 corresponding to an incoming dataunit to obtain one or more links corresponding to the composite next hopfor the data unit. Next hop table 630 may store the actual next hops.Thus, each next hop link in next hop logical chain table 620 may be usedto index next hop table 630 to obtain the next hop. Forwarding engine310 may then apply the composite function to chain together multiplenext hops and obtain the next hop rewrite for the data unit. Forexample, if incoming destination/route 610 refers to next hop links B,C, D in next hop logical chain table 620, forwarding engine 310 maylookup the next hops corresponding to links B, C, and D in next hoptable 630 and chain the partial rewrite results together to produce anoutput next hop rewrite that represents a chaining composition based onNH₂, NH₃, NH₄.

It can be appreciated that the architecture shown in FIG. 6 forforwarding table 315 is one example for implementing a chainedrepresentation of composite next hops in a forwarding table. Otherdesigns, such as other designs using other types of look-up tables orother structures may alternatively be used.

FIG. 7 is a flowchart of an exemplary process for forwarding data unitsbased on a chained representation of next hops. The exemplary process ofFIG. 7 may be performed by a forwarding engine 310 associated with eachinterface 200/230 of network node 140-x.

The exemplary process may begin with the receipt of a data unit (block700). An incoming data unit from network 130 may be received atforwarding engine 310 of interface 200/230 via, for example, a physicallink. One or more properties in a header of the received data unit maybe ascertained (block 710). For example, a destination address or routecorresponding to the data unit may be retrieved. A composite next hopmay be retrieved and assembled from forwarding table 315 based on achained representation of the next hops (block 720). As described above,the next hops may be obtained from forwarding table 315 by a lookupoperation that obtains links to multiple next hops and chains themtogether to form a composite next hop. The header of data unit 240 maynext be rewritten based on a rewrite string obtained from the assemblednext hop (block 730). The data unit may be forwarded to one or moreegress interfaces associated with outgoing links based on the assemblednext hops (block 740).

The chained assembly of next hops, as described above, can provide for anumber of advantages. For instance, by storing a single copy of a nexthop for multiple destinations/routes, memory requirements of theforwarding table can be reduced. Additionally, if there is a networkoutage or disruption causing a next hop to be changed by routing engine220, only a single copy of the next hop may need to be changed insteadof potentially needing to update many copies of the next hop.

Examples of Next Hop Chaining

To further illustrate certain of the concepts described above, exemplarysituations involving next hop chaining will next be described.

Assume a series of MPLS (multi-protocol label switching) packets arereceived by a node 140. FIG. 8A is a diagram illustrating exemplaryroutes associated with twelve exemplary MPLS packets, labeled as packetsone through twelve in FIG. 8A. Each packet may be associated with aningress label 805 and a next hop rewrite 810. Here, each next hoprewrite 810 is shown as one or two MPLS labels and an identification ofthe output port for the packet. Each MPLS label and output port maycorrespond to a next hop for the packet and are separated in FIG. 8Awith commas. As previously discussed, each next hop rewrite 810 may bedetermined by node 140 based on the next hop or next hops retrieved fromforwarding table 315.

If forwarding table 315 was implemented as a flat data structure, eachnext hop rewrite may be separately stored in forwarding table 315. Thiscan result in relatively large memory usage. Additionally, if a singlenext hop needs to be updated in forwarding table 315, each instance ofthat next hop may need to be located and updated. For example, assume aphysical link in the network fails and this failed physical link wasassociated with the label “10k_inner.” In this situation, each instanceof “10k_inner” may need to be located and updated in forwarding table315 to reflect new routes for the effected packets. In a largeforwarding table, locating and updating each instance of the label inthe forwarding table can take a significant amount of time.

FIG. 8B is a diagram illustrating an exemplary next hop table 830,similar to next hop table 630 (FIG. 6), constructed based on next hoprewrites 810. Eight rows, labeled A through H, are stored in next hoptable 830. Each next hop may represent an elemental next hop from thenext hop rewrite 810. In this MPLS example, each elemental next hop isshown as an MPLS label or egress port identification. Identical MPLSlabels that appear in different contexts may be separately stored innext hop table 830. For example, a MPLS tunnel label and VPN label maybe stored separately in next hop table 830 even if both labels have thesame value. In FIG. 8B, for instance, the label “10k” may be used indifferent MPLS contexts, denoted in next hop table 830 as “inner” and“outer,” and thus this label is stored in both row A and D in next hoptable 830. Similarly, the MPLS label “20k” is stored in both rows B andE and the label “30k” is stored in both rows C and F.

FIG. 8C is a diagram illustrating an exemplary next hop logical chaintable 840, similar to next hop logical chain table 640 (FIG. 6),constructed based on next hop rewrites 810. Each row in next hop table840 may include a logical list of the next hops for the routecorresponding to packets 1-12. The logical list of next hops may beexpressed, for example, as indexes pointing to the corresponding row innext hop table 830.

As another example of next hop chaining, consider the situation in whichEthernet data is sent over ATM (Asynchronous Transfer Mode). In such asituation, each rewrite may be an L2 rewrite of the form:<atm-cookie><atm-ether-address>. The “<atm-cookie>” portion of eachrewrite may be distinct for each route while the “<atm-ether-address>”may be the same for groups of routes.

FIG. 9A is a diagram illustrating three exemplary routes associated withEthernet over ATM transmission. Next hop rewrites 910 are shown for eachof three routes R1, R2, and R3. Each route includes a next hop labeled“cookie-if1” portion that is distinct for each route and a common“atm-ether-address” next hop.

FIG. 9B is a diagram illustrating an exemplary next hop table 930constructed based on next hop rewrites 910. Four rows, labeled A throughD, are shown in next hop table 930. Here, even though there are fourentries in next hop table 930 and only three in the “full” rewrite table910, memory usage can still be reduced as each entry in next hop table930 may take up less memory than each next hop rewrite 910.

FIG. 9C is a diagram illustrating an exemplary next hop logical chaintable 940, similar to next hop logical chain table 640 (FIG. 6),constructed based on next hop rewrites 910. Each row in next hop table940 may include a logical list of the next hops for routes R1, R2, andR3. The logical list of next hops may be expressed, for example, asindexes pointing to the corresponding row in next hop table 930.

As yet another example of next hop chaining, consider the situation inwhich next hop chaining is used to tabulate actions relevant to apartial next hop. With chaining of next hops, it may be possible toassociate a statistical action with only the partial next hop that isrelevant to the statistical action.

FIG. 10A is a diagram illustrating exemplary routes. Next hop rewrites1010 are shown for each of four routes Route1, Route2, Route3, andRouteM. Each route includes a next hop labeled one of NH1 through NHMand a next hop labeled NH0. Next hops NH1 through NHM may correspond toVPN labels and NH0 may correspond to the transport layer. Next hopegress E1 may correspond to the router egress re-write.

FIG. 10B is a diagram illustrating an exemplary next hop table 1030constructed based on next hop rewrites 1010. Six rows, labeled A throughG, are shown in next hop table 1030. Row A may represent an action notrelated to a header re-write, such as, in this case, a next hop actionthat calculates a byte count. In other words, row A may represent a“statistics gathering” next hop action. This next hop may be insertedinto the chain of next hop rewrites.

FIG. 10C is a diagram illustrating an exemplary next hop logical chaintable 1040, similar to next hop logical chain table 640 (FIG. 6),constructed based on next hop rewrites 1010. Each row in next hop table1040 may include a logical list of the next hops corresponding to routesRoute1, Route2, Route3, and RouteM. Additionally, in this example, alink to next hop A is also included in the routes. In this manner,statistics may be gathered as part of the next hop rewrite operation.

In contrast, without next hop chaining, because the partial next hops donot have an independent existence, it may not be possible to gatherstatistics relating to a single partial next hop. For instance, ifstatistics were desired for routes in which the NH1 next hop waspresent, it may be necessary to gather the statistics for all the routesin which “push NH0” appears. This can be an expensive operation if “pushNH0” appears in many routes.

Although many of the examples given above were in the context of MPLS,concepts described herein are not limited to MPLS. In general, anyrouting mechanism in which actions on the data units can be formulatedas partial updates may be implemented using next hop chaining Forexample, class-of-service (CoS) updates, statistics-gathering, policers,NAT updates, layer-2 rewrites, or other next hop actions may beimplemented using next hop chaining.

Regarding CoS, CoS updates may relate to the marking of certain dataunits with class of service tags. For example, all data units in which aparticular condition is satisfied (e.g., all data units having an evenvalued inner label) may be marked as belonging to a first class ofservice. In this case, next hop chaining could be used to link theaction “mark as class 1” with any route that satisfies this condition.

Regarding policers, policing actions may generally take an action basedon, for instance, a byte count associated with the policer. For example,assume that all data units having an inner label that satisfies acertain condition (e.g., the label is within a preset range) are be tolimited to a certain aggregate bandwidth threshold. In this case, nexthop chaining could be used to aggregate the number of bytescorresponding to the policer and perform a thresholding action, such asdropping data units when the threshold is exceeded.

An example of next hop chaining in which the next hop actions includepolicing and CoS related actions will now be discussed with respect toFIGS. 11A through 11D.

FIG. 11A is a diagram illustrating exemplary routes. In particular,assume there are 1000 possible routes, labeled as routes RT1 throughRT1000. Each route includes an inner label (one of labels 1001 through2000), an outer label (10,000), and an egress rewrite (fe000).

In this example, assume that data units matching routes which correspondto the odd inner label values (1001, 1003, . . . 1999) are to be markedwith a CoS label C1, and data unit matching routes which correspond tothe even inner label values (1000, 1002, . . . 2000) are to be markedwith the CoS label C2. Further, assume that that inner labels 1001through 1500 are to a set of VPN sites that are to be policed to anaggregate of B1 Mbps bandwidth and the other inner labels (1501 through2000) do not need to be policed. Further, assume that all the labelswitched paths going through label 10,000 should be policed at B2 Mbpsbandwidth.

FIG. 11B is a diagram illustrating routes RT1 through RT1000,represented using a flat, non-chained representation for the next hops.As shown in FIG. 11B, the next hop actions for each route includesactions for policing and CoS. Route 1, for instance, includes a policingaction that limits that bandwidth to the minimum of bandwidth B1 and B2(shown in FIG. 11B as “police(min(B1, B2))”). The policing action in theflat representation of the next hops, because it is flattened to asingle next hop action, may not be able to fully represent the desiredhierarchical policing based on two different labels. Instead, theminimum bandwidth value is policed. Additionally, route 1 includes theCoS action “mark C1.” With the non-chained representation shown in FIG.11B, the CoS marking action is repeated in each flat rewrite, resultingin the activity for marking the data unit as “mark C1” appearing 500times in the route rewrite table of FIG. 11B.

FIG. 11C is a diagram illustrating an exemplary next hop table, similarto next hop table 630 (FIG. 6), for the routes shown in FIG. 11A. Eachnext hop action is associated with a corresponding next hop index N1through N1006. In the table in FIG. 11A, the next hop actions “egressthrough fe000,” “push 10000,” “mark C1,” “mark C2,” “police to B1 Mbps,”and “police to B2 Mbps” each need to be only stored once.

FIG. 11D is a diagram illustrating an exemplary next hop logical chaintable, similar to next hop logical chain table 640 (FIG. 6). As can beseen, each chain of next hop actions includes one of “mark C1” (indexN3) or “mark C2” (index N4). Additionally, both policing actions(indexes N5 and N6) can potentially occur, in a chained manner, within aroute.

CONCLUSION

The foregoing description of embodiments described herein providesillustration and description, but is not intended to be exhaustive or tolimit the embodiments described herein to the precise form disclosed.Modifications and variations are possible in light of the aboveteachings or may be acquired from practice of the invention. While aseries of blocks has been described in FIG. 7, the order of the blocksmay vary in other implementations. Also, non-dependent blocks may beperformed in parallel.

Even though particular combinations of features are recited in theclaims and/or disclosed in the specification, these combinations are notintended to limit the invention. In fact, many of these features may becombined in ways not specifically recited in the claims and/or disclosedin the specification.

No element, act, or instruction used in the description of the presentapplication should be construed as critical or essential to theinvention unless explicitly described as such. Also, as used herein, thearticle “a” is intended to include one or more items. Where only oneitem is intended, the term “one” or similar language is used. Further,the phrase “based on” is intended to mean “based, at least in part, on”unless explicitly stated otherwise. The scope of the invention isdefined by the claims and their equivalents.

What is claimed:
 1. A method comprising: identifying, by a device andbased on a header of a data unit, a plurality of next hops associatedwith the data unit; obtaining, by the device and from a first datastructure of the device, references to information identifying theplurality of next hops, the information identifying the plurality ofnext hops being stored in a second data structure of the device, theinformation identifying the plurality of next hops, stored in the seconddata structure, identifying a plurality of actions to be performed onthe data unit; rewriting, by the device, one or more portions of theheader of the data unit to include the obtained references to theinformation identifying the plurality of next hops; and forwarding, bythe device and based on the obtained references to the informationidentifying the plurality of next hops, the data unit towards one ormore destinations, after rewriting the one or more portions of theheader of the data unit.
 2. The method of claim 1, where rewriting theone or more portions of the header of the data unit includes: rewritingat least one of layer 2 information or layer 3 information included inthe header of the data unit.
 3. The method of claim 1, furthercomprising: performing one or more of the plurality of actions on theheader of the data unit based on the obtained references to theinformation identifying the plurality of next hops.
 4. The method ofclaim 3, where performing the one or more of the plurality of actions onthe header of the data unit includes at least one of: performing anaction associated with a layer 2 portion of the header; or performing anaction associated with a layer 3 portion of the header.
 5. The method ofclaim 1, further comprising: obtaining, from the header of the dataunit, information identifying a route associated with the data unit,where the references to the information identifying the plurality ofnext hops are obtained, from the first data structure, based on theinformation identifying the route.
 6. The method of claim 1, whereforwarding the data unit includes: forwarding the data unit to one ormore egress interfaces of the device based on the obtained references tothe information identifying the plurality of next hops.
 7. A systemcomprising: a device to: obtain, from a first data structure of thedevice, links to information identifying a plurality of actions to beperformed on a data unit, the information identifying the plurality ofactions being stored in a second data structure of the device, thesecond data structure being different than the first data structure, andthe links to the information identifying the plurality of actions beingobtained based on information identifying a route of the data unit, theinformation identifying the route being obtained from a header of thedata unit; rewrite a portion of the header of the data unit to includethe obtained links to the information identifying the plurality ofactions; and forward, after the portion of the header of the data unithas been rewritten and based on the obtained links to the informationidentifying the plurality of actions, the data unit towards one or moredestinations associated with the data unit.
 8. The system of claim 7,where, when forwarding the data unit, the device is to: forward the dataunit to one or more interfaces of the device based on the obtained linksto the information identifying the plurality of actions.
 9. The systemof claim 7, where the device is further to: obtain, from the second datastructure, the information identifying the plurality of actions, basedon the obtained links to the information identifying the plurality ofactions, the plurality of actions being performed on the data unit basedon the obtained information identifying the plurality of actions. 10.The system of claim 7, where the data unit includes multi-protocol labelswitching (MPLS) information.
 11. The system of claim 7, where theportion of the header of the data unit includes at least one of layer 2information or layer 3 information, and where, when rewriting theportion of the header of the data unit, the device is to: rewrite the atleast one of the layer 2 information or the layer 3 information includedin the portion of the header of the data unit.
 12. The system of claim7, where each link, of the links, identifies a corresponding action ofthe plurality of actions.
 13. A device comprising: a routing componentto: obtain, from a first data structure of the device, references toinformation identifying a plurality of next hops, the informationidentifying the plurality of next hops being stored in a second datastructure, of the device, that is different than the first datastructure, the information identifying the plurality of next hops,stored in the second data structure, identifying a plurality of actionsto be performed on a data unit, the references to the informationidentifying the plurality of next hops being obtained based on a headerof the data unit, rewrite one or more portions of the header of the dataunit to include the obtained references to the information identifyingthe plurality of next hops, and forward, after the header of the dataunit has been rewritten and based on the obtained references to theinformation identifying the plurality of next hops, the data unit to oneor more destinations associated with the data unit.
 14. The device ofclaim 13, where, when obtaining the references to the informationidentifying the plurality of next hops, the routing component is to:obtain information identifying a route of the data unit based on theheader of the data unit, where the references to the informationidentifying the plurality of next hops are obtained from the first datastructure based on the information identifying the route.
 15. The deviceof claim 13, where the routing component is further to: obtain, from thesecond data structure, the information identifying the plurality of nexthops, based on the obtained references to the information identifyingthe plurality of next hops, the plurality of actions being performed onthe data unit based on the information identifying the plurality of nexthops obtained from the second data structure.
 16. The device of claim13, where, when rewriting the one or more portions of the header of thedata unit, the routing component is to: rewrite at least one of layer 2information or layer 3 information included in the header of the dataunit.
 17. The device of claim 13, where one or more actions, of theplurality of actions, are to be performed on the header of the dataunit.
 18. The device of claim 13, where the plurality of actions includeat least one of: an action associated with a layer 2 portion of the dataunit, or an action associated with a layer 3 portion of the data unit.19. The device of claim 13, where, when forwarding the data unit, therouting component is to: forward the data unit to one or more interfacesof the device, based on the references to the information identifyingthe plurality of next hops.
 20. The device of claim 13, where eachreference, of the references, identifies a corresponding next hop of theplurality of next hops.